When petroleum is found in subterranean formations or reservoirs, the first recovery stage, usually referred to as primary production, merely involves pumping or permitting the petroleum to flow to the surface of the earth through wells drilled into and in fluid communication with the subterranean reservoir. Primary production of petroleum is possible from the subterranean formations only if certain conditions exist. There must be an adequate concentration of petroleum in the formation, and there must be sufficient permeability or interconnected flow channels throughout the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the fluid. Primary production of oil ordinarily employs natural energy existing in the petroleum containing formation, usually in the form of an active, underlying or edge-waterdrive, solution gas, or a high pressure gas cap existing above the petroleum within the reservoir. When the naturally occurring energy source is depleted, or in the instance of those formations which do not originally contain adequate natural energy to permit a primary recovery phase, some form of supplemental recovery or enhanced recovery process must be applied to the reservoir. Supplemental or enhanced recovery is frequently referred to as secondary or tertiary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment. The injection of water into the formation to displace petroleum toward a spaced-apart production well from which it can be recovered to the surface of the earth, which is commonly referred to as waterflooding or secondary recovery, is the most economical and widely practiced form of supplemental recovery. Waterflooding is not an efficient oil recovery technique, however, since water and oil are immiscible and the interfacial tension between water and oil is quite high. This fact has been recognized in the literature pertaining to oil recovery, and many literature references describe the use of interfacial tension-reducing chemicals in the flood water injected in the formation, which results in displacing oil from flow channels not effected by passage of water alone. Organic sulfonates such as petroleum sulfonate are the most commonly described surfactant, although many references describe the use of more complex synthetic surfactants, including nonionic ethoxylated alkanols or alkylphenols, and sulfated or sulfonated, and ethoxylated alkanols or alkylphenols.
While the above described interfacial tension reducing chemicals, more commonly referred to as surfactants, reduce the interfacial tension between the displacing phase and the displaced phase, and therefore accomplish greater recovery from the portion of the formation through which the fluids pass than is possible using water alone, another problem which exists in any oil recovery method employing a displacement of petroleum by an injected fluid is concerned with poor vertical and horizontal conformance, or low volumetric efficiency of the displacement process. It is recognized that the mobility ratio between the injected fluid and displaced fluids is a major factor affecting the volumetric efficiency of a displacement process, and it is well recognized in the art that the incorporation of a viscosifying amount of a hydrophilic polymer such as polyacrylamide, partially hydrolyzed polyacrylamide, copolymers of acrylamide and acrylic acid, as well as biopolymers such as polysaccharides, in the fluid injected subsequent to injection of the surfactant containing fluid, improves the displacement efficiency of the oil recovery process.
Even employing the above-described fluids, enhanced oil recovery processes have usually not been entirely satisfactory under field operating conditions, principally because the amount of oil recovered is insufficient to justify the cost of the chemicals injected into the reservoir. The cause of the low recovery is complex and possibly related to many factors, but one cause is an adverse interaction which occurs between the surfactant fluid and the polymer fluid injected immediately thereafter. Whatever the cause, the technical and/or commercial failure of enhanced oil recovery processes making use of surfactant fluids and polymer fluids is well documented in the literature, and it can be appreciated that there is a substantial need for a cost-effective method for increasing the amount of oil recoverable by such processes.
It is an object of the present invention to provide an improved oil recovery method comprising injecting a slug of surfactant fluid followed by a slug of a polymer-containing fluid, by incorporating an additive in the polymer-containing fluid.